Routing devices within a network, often referred to as routers, maintain routing information that describe available routes through the network. Upon receiving an incoming packet, the router examines information within the packet and forwards the packet in accordance with the routing information. In order to maintain an accurate representation of the network, routers exchange routing information in accordance with one or more defined routing protocol, such as the Border Gateway Protocol (BGP).
Multi-Protocol Label Switching (MPLS) is a suite of protocols used to engineer traffic patterns within Internet Protocol (IP) networks. By utilizing MPLS, a source device can request a path through a network to a destination device, i.e., a Label Switched Path (LSP). An LSP defines a distinct path through the network to carry MPLS packets from the source device to a destination device. Each router along a LSP allocates a label and propagates the label to the closest upstream router along the path. Routers along the path cooperatively perform MPLS operations to forward the MPLS packets along the established path. A variety of protocols exist for establishing LSPs. For example, the Label Distribution Protocol (LDP), and the Resource Reservation Protocol with Traffic Engineering extensions (RSVP-TE).
Some implementations make use of Point to Multi-Point (P2MP) LSP in which a path is established through a network from a source device to multiple destination devices. P2MP LSPs are commonly used, for example, to distribute multicast data or to implement virtual private networks (VPNs). In the case of a P2MP LSP, one or more of the routers along the path may comprise branch routers located at points where the path divides. In addition to performing MPLS operations to forward the MPLS multicast packets along the path, the branch routers perform replication of the packets such that each branch of the P2MP LSP continues to carry copies of the multicast packets.
In generally, P2MP LSP construction follows a source-initiated signaling model in which the source device executes a label distribution protocol, such as RSVP-TE, to signal a different point-to-point LSP for each destination device (leaf node). The P2P LSPs, referred to as source-to-leaf (S2L) sub-LSPs, provide a label switch paths from the source device to a different, corresponding destination device. For example, the source device may signal the P2P sub-LSPs and combine the sub-LSPs to form the P2MP LSP. Techniques for forming a P2MP LSP using source-to-leaf sub-LSPs are described in RFC 4875, “Extensions to Resource Reservation Protocol—Traffic Engineering (RSVP-TE) for Point-to-Multipoint TE Label Switched Paths (LSPs),” IETF, May 2007, the entire contents of which are incorporated herein by reference.